Autonomous charge balancing of distributed AC coupled batteries with droop offset

ABSTRACT

A method and apparatus for autonomous charge balancing of an energy storage device of the microgrid. In one embodiment the method comprises obtaining, at a droop control module of a power conditioner coupled to an energy storage device in a microgrid, an estimate of a state of charge (SOC) of the energy storage device; introducing a bias, the bias based on (I) the estimate of the SOC and (II) a target SOC value for each energy storage device of a plurality of energy storage devices in the microgrid, to a droop control determination made by the droop control module; and generating, by the power conditioner, an output based on the droop control determination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/702,530, entitled “Autonomous Charge Balancing of Distributed ACCoupled Batteries with Droop Offset” and filed Dec. 3, 2019, which is acontinuation of U.S. patent application Ser. No. 15/369,876 entitled“Autonomous Charge Balancing of Distributed AC Coupled Batteries withDroop Offset” and filed Dec. 5, 2016 (now U.S. Pat. No. 10,511,178,Issued Dec. 17, 2019), which claims priority to U.S. Provisional PatentApplication No. 62/262,696, entitled “Autonomous Charge Balancing ofDistributed AC Coupled Batteries with Droop Offset” and filed Dec. 3,2015. Each of the aforementioned patent applications is hereinincorporated in its entirety by reference.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present disclosure relate generally to chargebalancing of batteries and, more particularly, to autonomous chargebalancing of AC coupled batteries in a microgrid.

Description of the Related Art

A conventional microgrid generally comprises at least one energygenerator, at least one energy storage device, and at least one energyload. When disconnected from a conventional utility grid, a microgridcan generate power as an intentional island without imposing safetyrisks on any line workers that may be working on the utility grid.

Droop control is one technique that may be used for operating energystorage and generation resources in a microgrid that is disconnectedfrom the utility grid. For several batteries in a microgrid having thesame droop characteristics, the batteries will share power equally amongeach other, or proportional to their power rating. Due to smalldifferences in chemistry, manufacturing tolerances, and the like, thebatteries won't charge and discharge at exactly the same rate. Inconventional microgrids that rely on communication between microgridresources when operating in an islanded state, such communication can beused to ensure that the charge among the batteries is balanced. However,if the microgrid communication is interrupted or disabled, thedifferences among the batteries will cause the batteries to drift apartand the charge among the batteries to become unbalanced.

Therefore, there is a need in the art for a technique for autonomouscharge balancing among batteries in a droop-controlled microgrid.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to autonomouscharge balancing among batteries in a droop-controlled microgrid asshown in and/or described in connection with at least one of thefigures.

These and other features and advantages of the present disclosure may beappreciated from a review of the following detailed description of thepresent disclosure, along with the accompanying figures in which likereference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a power system in accordance with one ormore embodiments of the present invention;

FIG. 2 is a block diagram of a power conditioner controller inaccordance with one or more embodiments of the present invention;

FIG. 3 is a block diagram of a droop control module in accordance withone or more embodiments of the present invention;

FIG. 4 is a block diagram of a droop control module in accordance withone or more embodiments of the present invention;

FIG. 5 is a block diagram of a DER controller in accordance with one ormore embodiments of the present invention; and

FIG. 6 is a block diagram of a component controller in accordance withone or more embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a power system 100 in accordance with oneor more embodiments of the present invention. This diagram only portraysone variation of the myriad of possible system configurations. Thepresent invention can function in a variety of environments and systems.

The power system 100 comprises a utility 102 (such as a conventionalcommercial utility) and a plurality of microgrids 150-1, 150-2, . . . ,150-X (collectively referred to as microgrids 150) coupled to theutility 102 via a utility grid 104. Through their connections to theutility grid 104, each microgrid 150 as a whole may receive energy fromthe utility grid 104 or may place energy onto the utility grid 104. Insome embodiments, coupling energy to a commercial utility grid isstrictly controlled by regulation and it is beneficial that themicrogrids 150 maintain or strive to maintain a zero energy outputpolicy. Each microgrid 150 is capable of operating without energysupplied from the utility 102 and may cover a neighborhood, a village, asmall city, or the like, as the term “microgrid” is not intended toimply a particular system size.

Although the microgrid 150-1 is depicted in detail in FIG. 1 anddescribed herein, the microgrids 150-2 through 150-X are analogous tothe microgrid 150-1. However, the number and/or type of variousmicrogrid components may vary among the microgrids 150.

The microgrid 150-1 comprises a plurality of microgrid members 152-1,152-2, . . . , 152-M (collectively referred to as microgrid members 152)coupled to a local grid 132 which in turn is coupled to the utility grid104 via an island interconnect device (IID) 134. The local grid 132 maybe a trunk of the utility grid 104 or it may be a specifically designedlocal grid for the microgrid 150-1.

The IID 134 determines when to disconnect/connect the microgrid 150-1from/to the utility grid 104 and performs the disconnection/connection.Generally, the IID 134 comprises a disconnect component (e.g., adisconnect relay) along with a CPU (not shown) and an islanding module(not shown) and monitors the utility grid 104 for failures ordisturbances, determines when to disconnect from/connect to the utilitygrid 104, and drives the disconnect component accordingly. For example,the IID 134 may detect a fluctuation, disturbance or outage with respectto the utility grid 104 and, as a result, disconnect the microgrid 150-1from the utility grid 104. The IID 134 may also disconnect the microgrid150-1 from the utility grid 104 when the microgrid 150-1 is eitheroverproducing energy or overloading the utility grid 104. Oncedisconnected from the utility grid 104, the microgrid 150-1 can continueto generate power as an intentional island without imposing safety riskson any line workers that may be working on the utility grid 104. In someembodiments, the IID 134 may receive instructions from another componentor system for disconnecting from/connecting to the utility grid 104.

The microgrid member 152-1 comprises a building 116 (e.g., a residence,commercial building, or the like) coupled to a load center 126 which maybe within or outside of the building 116. The load center 126 is coupledto the local grid 132 via a utility meter 120 and a local IID 122, andis further coupled to a distributed energy resource (DER) 106, agenerator 130, and one or more loads 118 for coupling power among thesecomponents. Although the microgrid member 152-1 is depicted in detail inFIG. 1 and described herein, the microgrid members 152-2 through 152-Mare analogous to the microgrid member 152-1. However, the number and/ortypes of various microgrid member components may vary among themicrogrid members 152.

The local IID 122 determines when to disconnect/connect the microgridmember 152-1 from/to the local grid 132 and performs thedisconnection/connection. For example, the local IID 122 may detect agrid fluctuation, disturbance or outage and, as a result, disconnect themicrogrid member 152-1 from the local grid 132. The IID 122 may alsodisconnect the microgrid member 152-1 from the local grid 132 when themicrogrid member 152-1 is either overproducing energy or overloading thelocal grid 132. Once disconnected from the local grid 132, the microgridmember 152-1 can continue to generate power as an intentional islandwithout imposing safety risks on any line workers that may be working onthe local grid 132. The local IID 122 comprises a disconnect component(e.g., a disconnect relay) for physically disconnecting from/connectingto the local grid 132. The local IID 122 may additionally comprise a CPU(not shown) and an islanding module (not shown) for monitoring gridhealth, detecting grid failures and disturbances, determining when todisconnect from/connect to the local grid 132, and driving thedisconnect component accordingly. In some embodiments, the local IID 122may receive instructions from another component or system fordisconnecting from/connecting to the local grid 132.

The meter 120 measures the ingress and egress of energy for themicrogrid member 152-1; in some embodiments, the meter 120 comprises theIID 122 or a portion thereof. The meter 120 generally measures realpower flow (kWh), reactive power flow (kVAR), grid frequency, and gridvoltage (referred to herein as the measured parameters). In certainembodiments these measured parameters may be communicated to a microgridmonitoring system (not shown) that monitors each of the microgridmembers 152.

The generator 130 is an energy generator, such as a diesel generator,that automatically increases or curtails energy output depending on theneeds of the microgrid member 152-1. The generator 130 comprises acomponent controller 128, described in detail further below with respectto FIG. 5 . The component controller 128 may optimize the operation ofthe generator 130 with respect to the microgrid member 152-1 and/or themicrogrid 150-1 (e.g., by generating control instructions for thegenerator 130); implement control instructions for operating thegenerator 130 (e.g., instructions received from another component orsystem); obtain data pertaining to the generator 130 (e.g., performancedata, operational data, or the like) which may further be communicatedto another component or system; or perform similar functions.

The loads 118 consume energy obtained via the load center 126 and may belocated inside of the building 116 or outside of the building 116. Someof the loads 118 may be “smart loads” that comprise a correspondingcomponent controller 128 for optimizing the utilization of energy (e.g.,disconnecting/connecting the smart load 118 when the grid isoverloaded/underloaded, modulating operation of smart loads 118, such asHVAC, pumps, and the like, as needed); implementing control instructionsfor the load 118 (e.g., instructions received from another component orsystem); obtaining data pertaining to the loads 118 (e.g., performancedata, operational data, and the like) which may further be communicatedto another component or system; or performing similar functions.

One or more of the smart loads 118 may be an energy storage componentthat stores energy received via the load center 126, such as a hot waterheater, an electric car, or the like. Such energy storage loads 118 mayfurther deliver stored energy to other loads 118 and/or the local grid132 as needed, where the energy storage and delivery is controlled bythe corresponding component controller 128.

The DER 106 comprises power conditioners 110-1 . . . 110-N, 110-N+1coupled in parallel to a bus 124 that is further coupled to the loadcenter 126. Generally the power conditioners 110 are bi-directionalpower conditioners and those power conditioners 110 in a first subset ofpower conditioners 110 are coupled to DC energy sources 114 (forexample, renewable energy sources such as wind, solar, hydro, and thelike) while the power conditioners 110 in a second subset of powerconditioners 110 are coupled to energy storage devices 112 as describedbelow. The combination of a DC energy source 114 and a correspondingpower conditioner 110 may be referred to herein as a DER generator. Inembodiments where the power conditioners 110 are DC-AC inverters, apower conditioner 110 and a corresponding energy storage device 112 maytogether be referred to herein as an AC battery 180; in embodimentswhere the power conditioners 110 are DC-DC converters, a powerconditioner 110 and a corresponding energy storage device 112 maytogether be referred to herein as a battery DC supply.

In the embodiment depicted in FIG. 1 , the power conditioners 110-1 . .. 110-N are respectively coupled to energy storage devices 112-1 . . .112-N to form a plurality of AC batteries 180-1 . . . 180-N,respectively. The AC battery power conditioners 110 convert AC powerfrom the bus 124 to energy that is stored in the corresponding energystorage devices 112, and can further convert energy from thecorresponding energy storage devices 112 to commercial power gridcompliant AC power that is coupled to the bus 124. An energy storagedevice 112 may be any suitable energy storage device having a “chargelevel”, such as a battery, flywheel, compressed air storage, or thelike, that can store energy and deliver the stored energy.

As further depicted in FIG. 1 , the power conditioner 110-N+1 is coupledto a DC energy source 114 (e.g., a renewable energy source such as wind,solar, hydro, and the like), forming a DER generator, for receiving DCpower and generating commercial power grid compliant AC power that iscoupled to the bus 124. In one or more embodiments, the DC energy source114 is a photovoltaic (PV) module. Although a single DER generator 190is depicted in FIG. 1 , other embodiments may comprise fewer for moreDER generators 190. In certain embodiments, multiple DC energy sources114 are coupled to a single power conditioner 110 (e.g., a single,centralized power conditioner). In one or more alternative embodiments,the power conditioners 110 are DC-DC converters that generate DC powerand couple the generated power to a DC bus (i.e., the bus 124 is a DCbus in such embodiments). In such embodiments, the power conditioners110-1 through 110-N also receive power from the DC bus and convert thereceived power to energy that is then stored in the energy storagedevice 112.

Each of the power conditioners 110 comprises a power conditionercontroller 140 (described in detail further below) having a droopcontrol module for implementing droop control techniques that allow thepower conditioners 110 to share the load in a safe and stable mannerwhen the microgrid member 152-1 is disconnected from the utility 102 orthe local grid 132.

The DER 106 comprises a DER controller 108 that is coupled to the bus124 and communicates with the power conditioners 110 (e.g., via powerline communications (PLC) and/or other types of wired and/or wirelesstechniques). The DER controller 108 may send command and control signalsto one or more of the power conditioners 110 and/or receive data (e.g.,status information, performance data, and the like) from one or more ofthe power conditioners 110. In some embodiments, the DER controller 108is further coupled, by wireless and/or wired techniques, to a mastercontroller or gateway (not shown) via a communication network (e.g., theInternet) for communicating data to/receiving data from the mastercontroller (e.g., performance information and the like).

In certain embodiments, the DER controller 108 comprises the local IID122 or a portion of the local IID 122. For example, the DER controller108 may comprise an islanding module for monitoring grid health,detecting grid failures and disturbances, determining when to disconnectfrom/connect to the local grid 132, and driving a disconnect componentaccordingly, where the disconnect component may be part of the DERcontroller 108 or, alternatively, separate from the DER controller 108.In some embodiments, the DER controller 108 may coordinate with thelocal IID 122, e.g., using power line communications.

Although the microgrid member 152-1 is depicted as having a single DER106 in FIG. 1 , in other embodiments the microgrid member 152-1 may haveadditional DERs. In one or more alternative embodiments, the DER control108 and the DER generators are absent from the microgrid member 152-1and the DER comprises only one or more AC batteries 180.

Each of the power conditioners 110, the generator 130, and any smartloads 118 are droop-controlled such that when the microgrid member 152-1is disconnected from the local grid 132 or the utility grid 104 (e.g.,using the IID 122 and/or the IID 134) and operating in an autonomousmode, these components employ a droop control technique for paralleloperation without the need for any common control circuitry orcommunication among the components.

In accordance with one or more embodiments of the present invention, theAC battery power conditioners 110 each employ a droop offsetproportional to the state of charge of the corresponding energy storagedevice 112 in order to maintain equal relative states of charge amongthe AC batteries 180 during autonomous operation. For each AC battery180, the droop offset is added to the power term within the powerconditioner's droop control module and varies (where a positive powercorresponds to power being exported from the DER) with the state ofcharge of the corresponding energy storage device 112, causing energy toflow from those AC batteries 180 having higher states of charge intothose AC batteries 180 with lower states of charge without disruptingthe fundamental stability of the droop control. Such operationautonomously drives the power storage devices 112 toward equal (orsubstantially equal) states of charge.

For example, if a particular AC battery 180 has a state of charge at50%, that AC battery 180 operates with its normalized droop function. Ifthat AC battery 180 has a state of charge that is below 50%, its droopwill be pushed down proportional to its deviation from 50%, forcing theAC battery 180 to run at a power level offset in the charging direction;i.e., if the battery 180 would have been charging, it is now charging ata higher rate, and if it would have been discharging, it will bedischarging at a lower rate. Conversely, if the state of charge is over50% the droop is slightly pushed up proportional to its deviation from50%, causing the AC battery 180 to begin operating at a power leveloffset in the discharging direction; i.e., if the battery would havebeen charging, it is now charging at a lower rate and if it would havebeen discharging, it will now discharge at a higher rate. As a result ofsuch a droop offset, those AC batteries 180 that have slightly differentstates of charge will have their droops offset slightly in differentdirections, resulting in a small amount of power that flows between theAC batteries 180 to equalize them.

The droop offset described herein may be employed in a variety ofdifferent types of droop control, including droop control for voltageforming inverters (as described below with respect to FIG. 3 ) and droopcontrol for current feeding inverters, which may also be referred to as“inverse droop” (as described below with respect to FIG. 4 ).

FIG. 2 is a block diagram of a power conditioner controller 140 inaccordance with one or more embodiments of the present invention. Thepower conditioner controller 140 comprises a transceiver 224, supportcircuits 204 and a memory 206, each coupled to a central processing unit(CPU) 202. The CPU 202 may comprise one or more conventionally availablemicroprocessors or microcontrollers; alternatively, the CPU 202 mayinclude one or more application specific integrated circuits (ASICs).The power conditioner controller 140 may be implemented using a generalpurpose computer that, when executing particular software, becomes aspecific purpose computer for performing various embodiments of thepresent invention. In one or more embodiments, the CPU 202 may be amicrocontroller comprising internal memory for storing controllerfirmware that, when executed, provides the controller functionalitydescribed herein.

The transceiver 224 may be coupled to the power conditioner's outputlines for communicating with the DER controller 108 and/or other powerconditioners 110 using power line communications (PLC). Additionally oralternatively, the transceiver 224 may communicate with the DERcontroller 108 and/or other power conditioners 110 using other type ofwired communication techniques and/or wireless techniques.

The support circuits 204 are well known circuits used to promotefunctionality of the CPU 202. Such circuits include, but are not limitedto, a cache, power supplies, clock circuits, buses, input/output (I/O)circuits, and the like.

The memory 206 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 206 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory206 generally stores the operating system (OS) 208, if necessary, of thepower conditioner controller 140 that can be supported by the CPUcapabilities. In some embodiments, the OS 208 may be one of a number ofcommercially available operating systems such as, but not limited to,LINUX, Real-Time Operating System (RTOS), and the like.

The memory 206 stores various forms of application software, such as apower conditioner control module 210 and a phase lock loop module 212for controlling, when executed, power conversion by the powerconditioner 110, and a droop control module 214 for employing, whenexecuted, droop control techniques as described herein. Thefunctionality of the droop control module 214 is described below withrespect to FIGS. 3 and 4 . The droop control module 214, when executed,operates in real-time or near real-time such that the correspondingenergy storage device 112 autonomous charge balances with respect to theremaining energy storage devices 112.

The memory 206 additionally stores a database 216 for storing datarelated to the operation of the power conditioner 110 and/or the presentinvention. In various embodiments, one or more of the power conditionercontrol module 210, the phase lock loop module 212, the droop controlmodule 214, and the database 222, or portions thereof, are implementedin software, firmware, hardware, or a combination thereof.

FIG. 3 is a block diagram of a droop control module 314 in accordancewith one or more embodiments of the present invention. As shown in FIG.3 , the droop control module 314 (an implementation of the droop controlmodule 214) is coupled between a PLL module 312 (an implementation ofthe PLL module 212) and a power conditioner control module 310 (animplementation of the power conditioner control module 210). The droopcontrol module 314 depicted in FIG. 3 provides droop control for voltageforming inverters—i.e., in those embodiments where the powerconditioners 110 comprise the droop control module 314, the powerconditioners 110 are voltage forming inverters.

The droop control module 314 comprises multipliers 320 and 322, adders332, 334, 340, 342 and 344, subtractor 348, and gain constantmultipliers 324, 326, 328, 330, 336, and 338. During autonomous modeoperation of the power conditioner 110, the grid voltage Ugrid and thecurrent being coupled to the grid by the power conditioner 110, Igrid,are fed to the PLL 312. Using the grid voltage Ugrid as a reference, thePLL 312 generates signals Id and lq, where Id represents the amplitudeof the portion of the grid current Igrid that is in-phase with the gridvoltage Ugrid and lq represents the portion of the grid current Igridthat is orthogonal to the grid voltage Ugrid, and couples the signals Idand lq to the multipliers 322 and 320, respectively. The PLL 312 furthergenerates the signal Ud which represents the peak value of thefundamental of the grid voltage Ugrid and couples the signal Ud to themultipliers 320 and 322.

The multiplier 320 multiplies ½ Ling to generate the signal Qrepresenting the reactive power component, and couples the signal Q tothe gain constant multipliers 324 and 328. The multiplier 322 multiplies½ Ud*Id to generate the signal P representing the real power component,and couples the signal P to the adder 344 and to the gain constantmultiplier 326.

The droop control module 314 receives a signal SOC representing anestimate of the current state of charge of the energy storage device 112coupled to the power conditioner 110. Generally, the signal SOC isacquired from a state of charge (SOC) estimator embedded in the energystorage device 112 (e.g., the signal SOC may be obtained from a batterymanagement unit of a battery 112 via an application programminginterface API), although in some alternative embodiments the SOCestimator may be part of the power conditioner 110. The received signalSOC is coupled to the subtractor 348.

The subtractor 348 further receives a signal SOC target that representsa target value for a state of charge for the energy storage device 112.The SOC target value is predetermined and is the same for each of thebatteries 112 within a particular microgrid member 152 in order for thecharge between the batteries 112 to autonomously equalize. In someembodiments, the SOC target value may be 50% state of charge such that,under normal conditions, all the batteries 112 within a particularmicrogrid member 152 are biased towards their 50% state of charge inorder to optimize the balance between the batteries 112 being able toabsorb excess power generated and to generate power when needed. In someother embodiments where the ability to power certain loads has a higherpriority than being able to store excess generated power, the SOC targetmay be set at a value much greater than 50%.

The output from the subtractor 348 is coupled to the gain constantmultiplier 346, which has a gain constant of −kb which essentiallydetermines the drift of the corresponding energy storage device 112.Generally the value of kb is extremely small to prevent the SOCestimation from significantly affecting the dynamic characteristics ofthe power conditioner 110. For example, if one energy storage device 112within a microgrid member 152 is undercharged with respect to the otherenergy storage devices 112 (e.g., the other energy storage devices are75% charged), it is desirable to have the one undercharged energystorage device 112 charge slightly faster or discharge slightly slowersuch that it slowly converges to the state of charge of the other energystorage devices 112. In some embodiments where the power rating for thepower conditioner 110 is ¼ of the KWH rating of the corresponding energystorage device 112 and the power conditioner 110 is 300 W conditioner,the value of kb may be 0.25 Watts/% for a 20% SOC difference and adesired 5 W difference in power between the energy storage devices 112(i.e., 5 W/20%). In some other embodiment, the value of kb may be evensmaller, for example 0.1 W/%.

The output from the gain constant multiplier 346 is a signalrepresenting an SOC-based droop offset that is proportional to the stateof charge of the corresponding energy storage device 112. The SOC-baseddroop offset signal is coupled to the adder 344 for addition to thepower term P. The resulting output signal from the adder 344, Poffset,is coupled to the gain constant multiplier 330.

The gain constant multipliers 324, 326, 328, and 330 have respectivegain constants X/Z, R/Z, −R/Z, and X/Z, where R, X and Z are impedanceterms that are generally matched to the grid impedance at their point ofcommon coupling, although they may be set using other techniques. Insome embodiments where the grid impedance is mostly resistive, a typicalvalue for X/Z may be on the order of 0.1, and a typical value for R/Zmay be on the order of 10.0. In other embodiments where the gridimpedance is mostly inductive, a typical value for X/Z may be on theorder of 10.0 and a typical value for R/Z may be on the order of 0.1.Generally, X/Z and R/Z ranges from 0.1-10.0, although the range may varydepending on the type of system to which the power conditioners 110 arecoupled.

The outputs from the gain constant multipliers 324 and 326 are coupledto the adder 332; the adder 332 generates the signal Q′ (whichrepresents the modified reactive power) and couples Q′ to the gainconstant multiplier 336. The gain constant multiplier 336 has a gainconstant −kq, which is a reactive power droop gain depending on the sizeof the power conditioner 110 (i.e., depending on the amount of reactivepower the power conditioner can deliver). In some embodiments where thesystem is a 240V system and the maximum reactive power delivery is 100var, kq may have a value of 0.24V/var to minimize the voltage drop to+/−10%.

The output from the gain constant multiplier 336 is coupled to the adder340, along with a signal U0 that represents the target nominal voltageof the system (e.g., 240V AC or 230V AC). The output signal from theadder 340 is a signal Usrc representing the peak AC operating voltagefor the power conditioner 110; the signal Usrc is coupled to the powerconditioner control module 310 for use by the power conditioner controlmodule 310 in generating the output from the power conditioner 110.

The outputs from the gain constant multipliers 328 and 330 are coupledto the adder 334; the adder 334 generates the signal P′ (whichrepresents the modified real power) and couples P′ to the gain constantmultiplier 338. The gain constant multiplier 338 has a gain constant−kp, which is a real power droop gain depending on the size of the powerconditioner 110 (i.e., depending on the amount of real power the powerconditioner can deliver). In some embodiments where the powerconditioner 110 is a 300 W power conditioner operating at a frequency of60 Hz, the value of kp is set at 0.01 Hz/W for a 5% droop. The outputfrom the gain constant multiplier 338 is coupled to the adder 342.

The output from the gain constant multiplier 338 is coupled to the adder342, along with a signal f0 that represents the target nominal frequencyof the system (60 Hz or 50 Hz). The output signal from the adder 342 isa signal fsrc representing the AC operating frequency for the powerconditioner 110; the signal fsrc is coupled to the power conditionercontrol module 310 for use by the power conditioner control module 310in generating the output from the power conditioner 110.

In some alternative embodiments, a computer readable medium comprises aprogram that, when executed by a processor, performs the steps describedwith respect to FIG. 3 for determining the power conditioner droopcontrol such that autonomous charge balancing of the energy storagedevices 112 is achieved.

FIG. 4 is a block diagram of a droop control module 414 in accordancewith one or more embodiments of the present invention. As shown in FIG.4 , the droop control module 414 (an implementation of the droop controlmodule 214) is coupled between a PLL module 412 (an implementation ofthe PLL module 212) and a power conditioner control module 410 (animplementation of the power conditioner control module 210). The droopcontrol module 414 depicted in FIG. 4 provides droop control for currentfeeding inverters—i.e., in those embodiments where the powerconditioners 110 comprise the droop control module 414, the powerconditioners 110 are current feeding inverters.

The droop control module 414 comprises multipliers adders 420, 422, 436,438, and 440, gain constant multipliers 424, 426, 428, 430, 432, 434,and 442, and subtractor 444. The gain constant multipliers 428, 430,432, and 434 have gain constants X/Z, −R/Z, R/Z, and X/Z, respectively,which are set as previously described with respect to FIG. 3 .

During autonomous mode operation of the power conditioner 110, the gridvoltage Ugrid and the current being coupled to the grid by the powerconditioner 110, Igrid, are fed to the PLL 412. Using the grid voltageUgrid as a reference, the PLL 412 generates signals Ud and f, whichrespectively represent the peak value of the fundamental of the gridvoltage Ugrid and the frequency of the grid voltage Ugrid, and couplesthe signals Ud and f to the respective adders 420 and 422. Additionally,the PLL 412 utilizes the current Igrid to calculate the portion of theoutput current that is in phase (Id) and quadrature (Iq) with the gridvoltage Ugrid for use in determining P and Q, although in otherembodiments P and Q may be determined by a different means, such as aDirect Fourier Transform (DFT) or a simple averaging scheme whereP=average (Ugrid[n]*Igrid[n]), S=average(Ugrid[n])*average(Igrid[n]),and Q˜sqrt(S²−P²).

In addition to the signal Ud, the signal U0, which represents the targetnominal voltage of the system (e.g., 240V AC or 230V AC), is coupled tothe adder 420. The output from the adder 420 is coupled to the gainconstant multiplier 424, which has a gain constant of ku, where the gainconstant ku is the inverse of the gain constant kp described above withrespect to FIG. 3 . The output signal from the gain constant multiplier424, Q′, is coupled to the gain constant multipliers 428 and 432.

In addition to the signal f, the signal f0, which represents the targetnominal frequency of the system (60 Hz or 50 Hz), is coupled to theadder 422. The output from the adder 422 is coupled to the gain constantmultiplier 426, which has a gain constant of kf, where the gain constantkf is the inverse of the gain constant kq described above with respectto FIG. 3 . The output signal from the gain constant multiplier 426, P′,is coupled to the gain constant multipliers 430 and 434.

The output signals from the gain constant multipliers 428 and 430 arecoupled to the adder 436 to generate the signal Q, which represents thereactive power component. The signal Q is coupled to the powerconditioner control module 410.

The output signals from the gain constant multipliers 432 and 434 arecoupled to the adder 438 to generate the signal P, which represents thereal power component. The signal P is coupled to the adder 440.Additionally, a signal representing an SOC-based droop offset that isinversely proportional to the state of charge of the correspondingenergy storage device 112 is coupled to the adder 440 for addition tothe power term P. The SOC-based droop offset is obtained as describedabove with respect to FIG. 3 ; i.e., an SOC estimate signal and an SOCtarget signal are coupled to a subtractor 444, where the output of thesubtractor 444 is coupled to the gain constant multiplier 442 togenerate the SOC-based droop offset. The gain constant multiplier 442has a gain constant or −kb as previously described. The resulting outputsignal from the adder 440, Poffset, is coupled to the power conditionercontrol module 410.

In some alternative embodiments, a computer readable medium comprises aprogram that, when executed by a processor, performs the steps describedwith respect to FIG. 4 for determining the power conditioner droopcontrol such that autonomous charge balancing of the energy storagedevices 112 is achieved.

FIG. 5 is a block diagram of a DER controller 108 in accordance with oneor more embodiments of the present invention. The DER controller 108comprises a transceiver 514, support circuits 504 and a memory 506, eachcoupled to a central processing unit (CPU) 502. The CPU 502 may compriseone or more conventionally available microprocessors ormicrocontrollers; alternatively, the CPU 502 may include one or moreapplication specific integrated circuits (ASICs). The DER controller 108may be implemented using a general purpose computer that, when executingparticular software, becomes a specific purpose computer for performingvarious embodiments of the present invention. In one or moreembodiments, the CPU 502 may be a microcontroller comprising internalmemory for storing controller firmware that, when executed, provides thecontroller functionality described herein.

The DER controller 108 generally communicates, via the transceiver 514,with the power conditioners 110 using power line communications (PLC),although additionally or alternatively the transceiver 514 maycommunicate with the power conditioners 110 using other types of wiredand/or wireless communication techniques. In some embodiments, the DERcontroller 108 may further communicate via the transceiver 514 withother controllers within the microgrid and/or with a master controller(not shown).

The support circuits 504 are well known circuits used to promotefunctionality of the CPU 502. Such circuits include, but are not limitedto, a cache, power supplies, clock circuits, buses, input/output (I/O)circuits, and the like.

The memory 506 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 506 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory506 generally stores the operating system (OS) 508, if necessary, of thepower conditioner controller 140 that can be supported by the CPUcapabilities. In some embodiments, the OS 508 may be one of a number ofcommercially available operating systems such as, but not limited to,LINUX, Real-Time Operating System (RTOS), and the like.

The memory 506 stores various forms of application software, such as aDER control module 510 for controlling operations pertaining to the DER106 (e.g., collecting performance data for the power conditioners 110,generating control instructions for the power conditioners 110, and thelike). The memory 506 additionally stores a database 512 for storingdata related to the operation of the DER 106. In various embodiments,one or more of the DER control module 510 and the database 512, orportions thereof, are implemented in software, firmware, hardware, or acombination thereof.

FIG. 6 is a block diagram of a component controller 128 in accordancewith one or more embodiments of the present invention. The componentcontroller 128 comprises support circuits 604 and a memory 606, eachcoupled to a central processing unit (CPU) 602. The CPU 602 may compriseone or more conventionally available microprocessors ormicrocontrollers; alternatively, the CPU 502 may include one or moreapplication specific integrated circuits (ASICs). The componentcontroller 128 may be implemented using a general purpose computer that,when executing particular software, becomes a specific purpose computerfor performing various embodiments of the present invention. In one ormore embodiments, the CPU 602 may be a microcontroller comprisinginternal memory for storing controller firmware that, when executed,provides the controller functionality described herein.

The support circuits 604 are well known circuits used to promotefunctionality of the CPU 602. Such circuits include, but are not limitedto, a cache, power supplies, clock circuits, buses, input/output (I/O)circuits, and the like.

The memory 606 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 606 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory606 generally stores the operating system (OS) 608, if necessary, of thecomponent controller 128 that can be supported by the CPU capabilities.In some embodiments, the OS 608 may be one of a number of commerciallyavailable operating systems such as, but not limited to, LINUX,Real-Time Operating System (RTOS), and the like.

The memory 606 stores various forms of application software, such as acomponent control module 610 for controlling, when executed, one or morefunctions of the corresponding component, and a droop control module 612for employing, when executed, droop control techniques for operating thecomponent.

The memory 606 additionally stores a database 612 for storing datarelated to the component. In various embodiments, one or more of thecomponent control module 610, the droop control module 612, and thedatabase 614, or portions thereof, are implemented in software,firmware, hardware, or a combination thereof.

When a microgrid member 152 is disconnected from the local grid 132and/or the utility grid 104, the power conditioner controllers 140 andthe component controllers 128 facilitate automatic control of thecorresponding components. For example, the power conditioner controlmodule 210 and the droop control module 214, when executed, facilitateautomatic control of the corresponding power conditioner 110; e.g., thepower conditioner control module 210 may monitor the power linefrequency and voltage at the corresponding power conditioner 110 toensure that the frequency and voltage stay within designated parameters.

By using such localized droop control, each component can autonomouslyoptimize its operation with respect to the microgrid member 152/overallmicrogrid 150. For example, for the generator 130, the componentcontroller 128 may optimize the generation of power; for smart loads118, and the component controller 128 may optimize the consumption ofenergy (e.g., by controlling the energy consumed by individual loadseither through throttling the flow or turning on and turning off variousloads at certain times).

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof is definedby the claims that follow.

The invention claimed is:
 1. A method for autonomous charge balancing ofa microgrid energy storage device, comprising: obtaining, at a droopcontrol module of a power conditioner coupled to an energy storagedevice in a microgrid, an estimate of a state of charge (SOC) of theenergy storage device; introducing a bias, the bias based on (I) theestimate of the SOC and (II) a target SOC value for each energy storagedevice of a plurality of energy storage devices in the microgrid, to adroop control determination made by the droop control module; andgenerating, by the power conditioner, an output based on the droopcontrol determination.
 2. The method of claim 1, wherein the bias isintroduced to a real power term of the droop control determination. 3.The method of claim 1, wherein the estimate of the SOC is obtained froman SOC estimator embedded in the energy storage device.
 4. The method ofclaim 1, wherein the energy storage device is a battery and the estimateof the SOC is obtained from a battery management unit of the battery. 5.The method of claim 1, wherein the power conditioner determines theestimate of the SOC.
 6. The method of claim 1, wherein, when optimizinga balance between the plurality of every storage devices absorbingexcess generated energy and providing energy when needed, the target SOCvalue is 50%.
 7. The method of claim 1, wherein, when powering one ormore loads has a higher priority than storing excess generated energy,the target SOC value is greater than 50%.
 8. Apparatus for autonomouscharge balancing of a microgrid energy storage device, comprising: adroop control module for providing droop control of a power conditionercoupled to an energy storage device in a microgrid, wherein the droopcontrol module obtains an estimate of a state of charge (SOC) of theenergy storage device and, based on (I) the estimate of the SOC and (II)a target SOC value for each energy storage device of a plurality ofenergy storage devices in the microgrid, introduces a bias in a droopcontrol determination used by the power conditioner in generating anoutput.
 9. The apparatus of claim 8, wherein the bias is introduced to areal power term of the droop control determination.
 10. The apparatus ofclaim 8, wherein the estimate of the SOC is obtained from an SOCestimator embedded in the energy storage device.
 11. The apparatus ofclaim 8, wherein the energy storage device is a battery and the estimateof the SOC is obtained from a battery management unit of the battery.12. The apparatus of claim 8, wherein the power conditioner determinesthe estimate of the SOC.
 13. The apparatus of claim 8, wherein, whenoptimizing a balance between the plurality of every storage devicesabsorbing excess generated energy and providing energy when needed, thetarget SOC value is 50%.
 14. The apparatus of claim 8, wherein whenpowering one or more loads has a higher priority than storing excessgenerated energy, the target SOC value is greater than 50%.
 15. A systemfor autonomous charge balancing of microgrid energy storage devices,comprising: a plurality of power conditioners in a microgrid, each powerconditioner of the plurality of power conditioners (i) coupled to adifferent energy storage device of a plurality of energy storagedevices, and (ii) comprising a droop control module for providing droopcontrol of the power conditioner, wherein the droop control moduleobtains an estimate of a state of charge (SOC) of the correspondingenergy storage device and, based on (I) the estimate of the SOC and (II)a target SOC value for each energy storage device of the plurality ofenergy storage devices, introduces a bias in a droop controldetermination used by the power conditioner in generating an output. 16.The system of claim 15, wherein the bias is introduced to a real powerterm of the droop control determination.
 17. The system of claim 15,wherein the estimate of the SOC is obtained from an SOC estimatorembedded in the corresponding energy storage device.
 18. The system ofclaim 15, wherein the corresponding energy storage device is a batteryand the estimate of the SOC is obtained from a battery management unitof the battery.
 19. The system of claim 15, wherein the powerconditioner determines the estimate of the SOC.
 20. The system of claim15, wherein, when optimizing a balance between the plurality of everystorage devices absorbing excess generated energy and providing energywhen needed, the target SOC value is 50% and wherein, when powering oneor more loads has a higher priority than storing excess generatedenergy, the target SOC value is greater than 50%.